1. Field of the Invention
The present invention relates to foamed well cement compositions, additives for foaming and stabilizing the compositions and methods of using the compositions.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casings and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string, e.g., casing, disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the exterior surfaces of the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
In carrying out primary cementing as well as remedial cementing operations in well bores, the cement compositions utilized must often be light weight to prevent excessive hydrostatic pressure from being exerted on formations penetrated by the well bore. Consequently, foamed cement compositions have often been utilized heretofore.
The typical non-foamed cement compositions utilized must have adequate pumping times before placement and relatively short set times after placement. If such a cement composition is slow to set, pressurized formation fluids can flow into and through the cement composition before and after it sets. Such an occurrence is attributable to the inability of the cement composition to transmit hydrostatic pressure during the transition time of the cement composition, i.e., the time during which the cement composition changes from a true fluid to a hard set mass. During the transition time, the initial hydration of the cement composition has begun and the composition starts to develop static gel strength. While the cement composition has little or no compressive strength, it becomes partially self-supporting which lowers the hydrostatic pressure exerted by the composition on pressurized fluid containing formations penetrated by the well bore. When the cement composition becomes partially self-supporting, volume reductions in the cement composition caused by hydration and fluid loss result in rapid decreases in the hydrostatic pressure exerted by the cement composition. This, in turn, allows pressurized formation fluids to enter the well bore and flow through the cement composition forming undesirable flow passages therein which remain after the cement composition sets. Thus, a further advantage of utilizing a foamed cement composition is that the gas in the composition is compressible. As a result of being compressed, the gas improves the ability of the composition to maintain pressure and prevent the flow of formation fluids into and through the cement composition during the transition time. Foamed cement compositions are also advantageous because they have low fluid loss properties.
While the heretofore used foamed cement compositions have included various surfactants known as foaming agents for facilitating the foaming of a cement slurry when a gas is mixed therewith and various other surfactants known as foam stabilizers for preventing the foamed cement slurries from prematurely separating into their slurry and gas components, various problems in the use of such surfactants have been experienced. For example, the particular foam stabilizer which must be used depends upon the particular temperature to which a foamed cement composition will be subjected. Further, the foaming agent used depends upon the particular components in the cement slurry. Also, the quantities of the foaming agent and foam stabilizer will vary depending upon the particular types of foaming agent and foam stabilizer used. In addition, upon mixing the foaming agent and the foam stabilizer with water, gelation sometimes occurs. Finally, the presence of the heretofore used foaming agents and foam stabilizers often lowers the compressive strengths of foamed cement compositions after they set.
Thus, there are needs for improved foamed well cement compositions, additives for foaming and stabilizing the compositions and methods of using the compositions which obviate the above described problems.